The invention relates to a high-voltage cable which comprises a conductor and a polythene containing insulation sheath provided around the conductor and comprising an agent to prevent or impede the formation or the growth of watertrees.
A high-voltage cable generally comprises a central copper conductor having a diameter which may vary from a few millimeters to a few centimeters. Around the conductor an internal semiconductor screen is present which, for example, consists of polythene (polyethylene) or a copolymer of polythene, for example, the copolymer of ethylene and vinyl acetate in which soot has been dispersed. The internal semiconductor screen is covered and surrounded by a polythene containing insulation sheath. The insulation sheath may consist entirely of polythene (PE), of crosslinked polythene (XLPE) or of a copolymer of ethene and, for example, another alkene or a diene, such as the copolymer of ethene and propene (EPM) or the terpolymer of ethene, propene and a diene (EPDM). The abbreviations in brackets are internationally used indications of the type of the relevant insulation material. Around the insulation sheath there is provided an external semiconductor screen of usually the same composition as the first semiconductor screen. Around the external semiconductor screen there is present a conductive layer of, for example, a metal foil, such as a copper foil, which in turn is covered with a layer of a synthetic resin, over which the usual reinforcement and finally an outer cover of a synthetic resin of for example PVC, is provided.
In particular the high-voltage cables insulated with polythene (PE) or cross-linked polythene (XLPE) are widely used nowadays.
When the high-voltage cable is used, small hair line cracks occur with time under the influence of moisture in the polythene containing insulation sheath which enlarge in the form of a tree with numerous branches. Such branched breakage forms are termed watertrees. The phenomenon is internationally known as "watertreeing" or "electrochemical treeing". It will be obvious that the watertrees have a detrimental influence on the insulation qualities of the polythene containing sheath and may give rise to breakdown.
The formation and the growth of watertrees may be inhibited by ensuring that no moisture is incorporated in the polythene containing insulation material during the manufacture of the cable and then furthermore sealing the cable in a moisture-tight manner from the atmosphere, for example, by providing the finished cable with a metal cover such as a lead or aluminum sheath. It will be obvious that such a cable is very expensive and in addition becomes less loadable thermally.
It is known that the growth of watertrees in polythene containing insulation materials is retarded when the insulation material contains acetophenone. This is the decomposition product of dicumylperoxide which usually is used as an auxiliary substance in cross-linking polythene. The acetophenone is rather volatile and in the long run disappears from the insulation material so that the delay of the growth of watertrees only has a temporary character.
It is furthermore known from German Offenlegungsschrift 2,537,285 that the growth of watertrees is suppressed when an electrolyte is added to the polythene containing insulation sheath. Here, however, the possibility of breakdown of the insulation is increased, a result which is rather to be avoided.